Methods for determining the intrinsic kinetic characteristics of irreversible adsorption processes

Abstract

To benchmark the performance of a different of catalytic materials it is essential to know the adsorption characteristics such as the kinetic order of adsorption, the apparent rate constant and the total number of active sites. It is advantageous to normalize the reaction rate to the number of active sites; a quantity commonly referred to as the turnover frequency (TOF). Methods such as chemisorption and SSITKA (steady state isotopic transient kinetic analysis) are conventionally used to quantify the number of active sites, both have advantages and drawbacks. The TAP (Temporal Analysis of Products) pulse response technique provides a distinct method for precise (10 nmol) quantification of active sites at elevated temperatures. Using irreversible reaction processes, mathematical techniques for analysis of pulsed active site titration are described herein. Whereas previous methods simply consider the total capacity for gas adsorption, these new methods take into account the dynamics and interdependence of the conversion, the number of available sites and the adsorption rate as they change over the course of the titration experiment. Moreover, these methods enable an independent fitting of the kinetic order and stoichiometry of the adsorption process directly from experimental data. In comparison with experimental data for the incremental oxidation of a reduced platinum, we found the analytical method of linear rectification was most robust and efficient. In fitting the kinetic order to experimental data collected between 125 and 175 °C we found a clear indication over the range of 45–95% conversion of a second order rate dependence and 2:1 balance between active sites and O2.